It's probably just a coincedence that it happened there.
There is nothing they could do to the valve with out pulling it apart unless they dropped something into the injector hole when the injector was out or something in the combustion chamber when the plug was out.
It's not something that would normally happen, but i suppose it is possible.
You would not be able to tell until the heads came off
I'm not sure what the cams are like in the EF XR8, but it may also be possible that you have a worn cam lobe which would explain it slowly starting the problem and then it sped up towards the end.
Another possiblilty may be a burnt valve, if you have LPG this may be more of a possibility.
With older cars it was possible to do damage by over revving the motor, but with modern cars and their computers, the motor can not be revved to do the damage as the factory rev limiter cuts in.
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